1. Technical Field of the Invention
The present invention relates to a process for producing a Metal-Oxide-Semiconductor (MOS) transistor whose gate incorporates a silicide-type chemical compound.
2. Description of Related Art
An MOS transistor with a fully silicided gate has well-known advantages over an MOS transistor with a gate made of polysilicon or made of a metal such as tungsten or molybdenum, or made of tantalum carbide, etc. In particular, a fully silicided gate has no electron depletion zone in contact with the gate dielectric of the transistor, so that the apparent gate capacitance is lower. Furthermore, when the gate is made of a silicide, it is definitively produced at the end of the transistor production process. The silicide material therefore incorporates much fewer impurities than another gate material formed at the start of the transistor production process, such as tungsten, molybdenum or tantalum carbide. Moreover, the silicide does not have to undergo the activation annealing of the source and drain zones, which is carried out at a temperature of up to about 1000° C. Thus, the thermal stability requirements imposed on a gate silicide compound are much less restrictive than those imposed on other materials that have to withstand such temperatures. The electronic properties of the gate, among which are included the electrical resistivity and the work function, are therefore better controlled.
To obtain a gate with few impurities and to have fewer constraints regarding the thermal stability of the gate material (at high temperatures of about 1000° C.), it is also known to produce a transistor with a false gate and then, at the end of the transistor production process, to replace the false gate with a definitive gate. Thus, the definitive material of the gate is not contaminated by impurities during production of the transistor. However, the use of a false gate is particularly complex, especially owing to the chemical-mechanical polishing of the first dielectric level, which must be stopped just at the top of the gate, the removal of the false gate and the formation of the definitive gate material only at the location thereof.
A transistor with a fully silicided gate is therefore particularly advantageous, especially because it is simple to produce and because the electronic properties of the gate can be well controlled. However, the following difficulty arises during its production. The silicide compound used for the gate may have several different chemical phases. In particular, there may be several stoichiometric ratios, that is to say several values of the metal/silicon atomic ratio. As an example, at least two compounds of the nickel silicide type exist, and a gate obtained by nickel silicidation has in general a first phase of formula Ni2Si in an upper part of the gate and a second phase of formula NiSi in a lower part of the gate. The boundary separating these two phases is of irregular, and in general uncontrolled, shape so that the two phases—Ni2Si and NiSi—may be simultaneously in contact with the gate dielectric, or else only the NiSi phase is in contact with the gate dielectric. But, certain electrical characteristics of MOS transistors, among which is included the transistor switching voltage, depend on the work function of the gate and therefore on the chemical phase of the gate material that is in contact with the gate dielectric. For a MOS transistor with a nickel silicide gate, these characteristics therefore vary unintentionally, for example between two transistors located at different points on the same silicon wafer, or between two transistors produced on different wafers, even though identical process parameters were used for both wafers. In particular, certain transistors may have a switching voltage similar to that of a transistor with an n-doped or p-doped polysilicon gate, while others may have a switching voltage corresponding to a metal gate, the Fermi level of which lies within the bandgap of the substrate (in what are called “mid-gap” transistors).
To prevent the boundary separating the two phases, Ni2Si and NiSi, from reaching the gate dielectric, MOS transistors are produced with a thick nickel silicide gate. For example, the gate has a thickness of the order of 100 nanometers. In this case, only the NiSi phase is in contact with the gate dielectric layer, so that the resulting transistors have a constant and well-defined switching voltage. Furthermore, the gate has a low electrical resistivity, compatible with use of the transistor in the microwave range.
However, portions of silicide compound are also needed in the source and drain zones of the transistor in order to obtain low electrical contact resistances in these zones. These silicide portions of the source and drain zones must be thin in order to avoid leakage currents from appearing in the electrical junctions between the source and the channel of the transistor on the one hand, and between the channel of the transistor and the drain on the other. The formation of the silicide compound for the gate then has to be dissociated from the formation of the silicide compound for the source and drain zones of the transistor, so as to produce a sufficiently thick silicide gate, but thin silicide portions in the source and drain zones.
However, such a dissociation between the formation of the silicide compound for the gate on the one hand, and the formation of the silicide compound for the source and drain zones on the other, requires adding steps to the transistor production process. In particular, at least one CMP polishing step is needed and/or at least one masking step. Such additional steps are disadvantageous, as they increase the risk of introducing impurities and/or particles into certain parts of the transistor. Furthermore, during an additional CMP polishing step, the repeatability and uniformity of the polishing carried out may be difficult to control. Likewise, an additional photolithography step introduces alignment difficulties during masking. In general, dissociating the formation of the silicide compound for the gate on the one hand and that for the source and drain zones on the other, results in an increase in the manufacturing costs of an electronic circuit that incorporates transistors obtained in this way.
There is accordingly a need for a process for producing an MOS transistor with a fully silicided gate that does not have the abovementioned drawbacks.